SYNTHESIS OF ANGULARLY ANNULATED TETRAQUINANES BY PHOTO-THERMO METATHESIS OF DIASTEREOMERICALLY PURE ADDUCTS BETWEEN BENZOQUINONE AND TETRAHYDROPENTALENES

Synthesis of Angularly Annulated Tetraquinanes by Photo‐Thermo Metathesis of Diastereomerically Pure Adducts Between Benzoquinone and Tetrahydropentalenes A retrosynthesis is presented for an angularly fused tetraquinane, the central step of which is the photo‐thermo metathesis of a tetrahydropentalene ‐ benzoquinone adduct. A sequence of Michael addition/condensation reaction gives the 3‐methyl‐ l‐phenyl‐1,2‐dihydropentalene (2) in 70% yield from benzalacetone and cyclopentadiene. Reaction with lithium aluminum hydride lead to a 1.0: 1.0:0.8 mixture of the 1,2,3,4‐tetra‐hydropentalenes 4a and 4b and the 1,2,3,5tetrahydropentalene 5. This reaction is highly diastereoselective (d.s. > 96%) and provides the cis compounds exclusively. Diels‐Alder cycloadditon of 4a, b and 5 to benzoquinone also proceeds highly stereoselectively. The adducts 7a, b and 8 may be separated by fractional crystallization. Their relative configuration can be determined by means of 2D‐COSY and NOE experiments. The endo arrangement is demonstrated by the photoreactivity of the cycloadducts. Gas‐phase thermolysis (700°C/0.05 Torr) of the cage molecules 9a and 9b (from irradiation in benzene) gives the angularly annulated tetraquinanes 10a and 10b in 95 and 88% yield, respectively. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim